A. Field of the Invention
This invention relates to the field of programmable load controllers for residential electric utility customers.
B. Background Art
One of the most serious problems confronting electric utility companies today is the great variance in total electrical demand on a network between peak and off peak times of the day. The so-called peak demand periods or load shedding intervals are periods of very high demand on the power generating equipment where load shedding may be necessary to maintain proper service to the network. These occur for example during hot summer days occasioned by the widespread simultaneous usage of electric air conditioning devices. Typically the load shedding interval may last many hours and normally occurs during the hottest part of the day. Peaks may also occur during the coldest winter months in areas where the usage of electrical heating equipment is prevalent. The increasing use of electrical space heaters contributes to this problem. In the past, in order to accomodate the very high peak demands, electric utility companies have been forced to spend tremendous amounts of money either in investing in additional power-generating capacity and equipment, or in buying so-called peak power from other utilities which have made such investments.
Typically the utilities have met this fluctuating electricity demand by using three types of generation. Base load plants (usually nuclear or coal fired) provide nearly constant output. These units are the most efficient, and the most economical to operate. Intermediate plants follow the slow variations in the daily electricity demand, and are consequently more expensive to run. Peaking plants are the least efficient and the most costly of all. Peaking plants are usually gas- or oil-fired turbines that can be started quickly to meet high demands for short periods. Times when a utility must resort to using more of the inefficient and uneconomical generation are termed "on-peak" or "peak times".
The power a customer uses is actually what causes the utility problems, since the utility must have enough "capacity" to meet the customer's electricity demand at all times. In order to encourage customers to reduce demand and avoid building new plants, most utilities include a demand or capacity charge in the rate. If the consumer does not reduce its demand this charge helps pay for new generation. The highest electricity demand in a billing period is compared to a demand norm and the credit or penalty is calculated. Once a penalty has been imposed for a billing period it cannot be removed by controlling demand below the norm during the remainder of the billing period. Thus there is no additional economic advantage to controlling below the demand level which invoke the penalty. This type of demand charge has been common in industrial rates for many years, but has recently been introduced for residential purposes. It is based upon energy use over a predetermined period of time (conceptualized as a sliding window), rather than upon instantaneous use. There is thus no additional economic advantage to controlling spikes in demand, no matter how high they are, if their duration is short enough that they do not raise the demand over the fifteen minute sliding window enough to cause an additional penalty.
In the prior art several basic strategies and devices have been utilized for load shedding in order to limit the peak power demands on the power generating capacity of electric utility companies. One such mode involves sending signals from the utility to disconnect selected electric loads. While this direct control of power consumption by the utility achieves usage cutbacks, the lack of flexibility may inconvenience the user.
An alternative is installing load shedding devices at the point of consumption. This provides greater flexibility and control. Several such devices have been available but none has the ability to adjust the control point upward as the billing period progresses. This presents difficulties for users that have priority loads which must be energized on an as needed basis. An overrideable, fixed control point allows all priority loads to be energized. However, the minimum level necessary to accomodate these loads may unavoidably result in a penalty charge. Subsequently, there is no economic advantage to shedding non-priority loads to stay below this minimum level. However, a situation may arise in which the control point has been overridden in order to allow all priority loads to be energized but, currently, not all priority loads are in use and non-priority loads are deenergized to stay below the control point which has already been exceeded. Thus, the non-priority loads would be shed unnecessarily.